The "Russkij.htm" file does not display correctly on the Opus - it just shows as random ASCII junk. I suspect that it could be made to display correctly by manually selecting a suitably-encoded font.

That's correct; the real problem with Russian/Cyrillic text is that there are something like 4 widely-used encodings out there, and it's a pain to figure out which one every single text uses, ensure you save it with correct encoding, find fonts that will work with that encoding... the HTML character conversion is much simpler, even for fairly long texts... plus you get the added bonus of not having to change fonts to view it.